Tandem solar cells, also known as multijunction, cascade, or multi-bandgap solar cells, contain two or more subcells, with each subcell containing a PN junction designed to convert light into electrical power from a different part of the solar spectrum. Subcells are made from various semiconductor materials. In the final solar cell, the subcells are vertically stacked, in order, from the subcell with the highest bandgap in the uppermost position to the subcell with the lowest bandgap in the lowest position in the stack. The highest bandgap subcell receives the incident sunlight first, absorbing mostly short wavelength light, while its material allows longer wavelength light to pass through virtually unabsorbed. Each subsequent subcell in the tandem stack then absorbs some remaining light, passing on the longer wavelengths of the light. The purpose of these multiple subcells, each of a different bandgap, is to more efficiently utilize the solar spectrum and increase the solar to electrical power conversion efficiency compared to a single junction solar cell.
The semiconductor materials used for the substrate or support substrate and the various stacked subcells (e.g. top, middle, bottom cell in a three junction tandem cell) epitaxially grown on the substrate should have nearly the same (e.g. within a few thousand parts per million) semiconductor crystalline lattice constant (e.g. 5.65 angstroms for gallium arsenide based cells) to avoid crystal defects (e.g. dislocations) which may form to relieve the mechanical strain caused if there is lattice mismatch between subcells. These dislocation defects often act as minority carrier recombination sites, which degrade the power conversion efficiency of the solar cell. The choices of materials for the support substrate and the various component subcells (top, middle, etc.) of the tandem are thus limited to those closely lattice-matched with each other. Useful tandem cells, known as metamorphic cells, can be made despite the presence of significant lattice mismatch, if a greater benefit can be achieved (e.g. freedom to match bandgaps of subcells to maximize and equalize the electrical current generated in each subcell) than that given up by the introduction of the dislocations.
One approach to this problem is to use bifacial, tandem solar cells where the subcells can be placed on both sides of a substrate to isolate the effects of lattice mismatch to cells on one side of the substrate. U.S. Patent Publication US2006/0162768 A1, by Wanlass et al. covers the general case of a transparent semiconducting substrate with subcells grown on each side. In comparison with Wanlass, the inventions claimed here are related to particular non-obvious process sequences used to grow the subcells on each side of the substrate. In US 2005/0056312 A1, Young et al. describes subcells on opposite sides of an insulating glass substrate and in U.S. Pat. No. 4,289,920 Hovel describes subcells on opposite sides of an insulating ceramic substrate. Both of these patents, although describing a bi-facial growth, do not use epitaxial growth on a semiconducting substrate on which our particular invention claims are based.
Currently inverted metamorphic processes are being used to make high efficiency tandem solar cells but those processes involve substrate bonding and epitaxial lift-off which are lower yield when used over large substrate areas. “40.8% Efficient Inverted Triple-Junction Solar Cell With Two Independently Metamorphic Junctions” by J. F. Geisz et al., Applied Physics Letters 93, 123505 (2008).